The present invention is related to the field of rotary optical encoders used to sense the rotational position of rotatable objects such as servo motors.
Position feedback is needed for closed loop control of the rotational position of motors including reciprocating motors. A variety of position sensor technologies have been employed, including optical position detectors which operate by modulating light in some manner as a function of rotational position and converting the detected modulation into corresponding angular position values. In motor applications, optical position detectors are commonly located at one axial end of the rotatable shaft of the motor. A component such as a patterned reflective disk may be coupled to the end of the rotatable shaft, and adjacent encoder elements operate to generate a light signal and detect light reflected from the disk. The modulation may be accomplished, for example, by employing a pattern of reflective and non-reflective areas on the disk such that the pattern of reflected light is indicative of the rotational position of the disk and therefore also of the motor shaft.
One problem experienced with optical position detectors according to the above arrangement is a certain type of inaccuracy resulting from eccentric motion or “wobbling” of the motor shaft as it rotates (also referred to as radial run-out) or from improper radial alignment of the reflective disk and the shaft. The eccentric motion of the disk imparts an eccentric component to the reflected light pattern. Part of this eccentric component is incorrectly perceived by the optical position detector as shaft rotation, and thus the output of the optical position detector includes a spatially periodic error component.
It has been known to address the problem of eccentricity-induced error by employing a pair of optical position detectors at diametrically opposite positions of the encoder disk (i.e., separated by 180 degrees). The analog outputs of the optical position detectors are summed such that the effect of eccentricity within a certain range is completely cancelled. While this configuration can effectively eliminate eccentricity errors, it still suffers from two drawbacks. First, differential motion between the optical position detectors in a direction perpendicular to the diameter line between them is perceived as rotary motion. Such relative motion can occur if the mounting structure or the components of the position detectors move in response to temperature changes. The second disadvantage is difficulty in aligning the position detector. The two separate position detectors must be aligned so that corresponding analog output signals are in phase with each other, otherwise the modulation of the summed signals is diminished. A low modulation signal reduces the signal-to-noise ratio and limits the interpolated resolution of the position detector.